Inductors are commonly required in small, low volume, low weight circuits such as buck converter circuits used for dc to dc converters. The conventional inductors used in such circuits are usually large and thus occupy considerable space on printed circuit boards. It would be desirable to manufacture inductors which can be easily integrated with semiconductor circuit devices such as MOSFETs, integrated circuits and other passive elements such as resistors and capacitors without using substantial printed circuit board area.
In the recent years, embedded inductors have been developed to address the need for having a small inductor. A well known structure for an embedded inductor is a spiral-shaped inductor embedded in a magnetically permeable material. Spiral shaped inductors embedded between substrate/pre-preg materials or permalloy loaded epoxies are examples of such known embedded inductors. Permalloy is a ferromagnetic material of high magnetic permeability which is responsible for increasing the device inductance.
Results from experiments drawn between spiral inductors embedded between substrate/pre-preg materials and those embedded between permalloy loaded epoxies have shown that coils surrounded by the permalloy loaded epoxy compounds have up to 10% higher inductance compared to coils surrounded by substrate/pre-preg materials. Thus, although permalloy could be used to reduce the size of the spiral inductor in an embedded inductor, the improvement in inductance is not significant.
It is, therefore, desirable to have an embedded inductor with improved inductance per unit area.